1. Field of the Invention
The present invention relates generally to data storage method and apparatus for recording and retrieval of information on and from a data storage medium in the form of a magnetic, optical or other memory disk having a multiplicity of recording tracks. More particularly, the invention is concerned with a technique for determining alternative tracks used in place of defective tracks when an external device commands the apparatus to record information on the defective tracks.
2. Discussion of the Prior Art
It is ideal that a data storage medium such as a magnetic or optical memory disk is completely free from defects on their recording surfaces. However, the present technology for fabricating such memory disks does not permit complete elimination of the defects. For example, a magnetic disk inevitably suffers from defects due to low surface accuracy of a substrate, inconsistency in connection with coatings of the substrate surfaces with a magnetic material, or pin holes in the magnetic coatings. For maintaining the yield of the product at a sufficiently high level, the manufacturers are therefore forced to offer such data storage media as acceptable products even if they have some limited number of defects.
A conventional approach to enable the use of such partially defective data storage media is to use a specific track such as track #0 for prerecording defective-track data indicative of the locations of defective tracks. For instance, if a memory disk has defects in tracks #200 and #250, the prerecorded defective-track data represents alternative tracks #304 and #305 for the defective tracks #200 and #250, respectively. When a data storage apparatus using such a memory disk is turned on, the defective-track data is transferred to a controller, which selects the alternative track #304 when the defective track #200 is accessed by an access signal from an external host computer. Similarly, the controller selects the alternative track #305 when the defective track #250 is designated by the host computer. This arrangement makes it possible to use a data storage medium which has some defects, and eliminates a need for the host computer to check the recording tracks for any defects, after the start of the data storage apparatus. Therefore, the throughput time may be diminished.
In the conventional arrangement described above, however, the manufacturer of the data storage apparatus loaded with partially defective data storage media must provide a user such as a computer manufacturer with a list of the defective tracks, so that the user may prerecord the alternative-track data on an appropriate track (such as track #0), or store the data in a read-only memory (ROM) built in a controller board, for automatic selection of alternative tracks in place of the defective tracks. Further, if the data storage apparatus is removed from one computer and installed in another computer, or if the format of the storage media is changed, it is necessary to re-record the alternative-track data on the media or change the previously stored data in the memory, requiring extra time.
Moreover, the above method does not allow the use of a data storage medium which has a defect in the special track on which the alternative-track data is stored. That is, if the special track of the medium has a defect, this means that the medium should be rejected.
Generally, an operation to write or read information on or from a storage medium proceeds from one track to the next. Therefore, if the radially innermost or outermost track and the adjacent tracks are used as alternative tracks for defective tracks, as indicated above, the read/write head must be moved to the radially inner or outer alternative track each time the defective track is externally accessed. This results in increased time for positioning the read/write head on the alternative tracks, leading to increased throughput time of the data storage apparatus.